The complex and diverse functions of the brain depend on the unique properties of neural circuits formed by various subtypes of neurons with distinct molecular and/or electrical properties. Furthermore, many neurological and psychiatric disorders are often due to the dysfunction of specific subsets of neurons or neural circuits. Thus, elucidating the unique roles of each subtype of neurons in shaping circuitry function is critical t our understanding of both normal and abnormal brain functions. Genetic tools that incorporating spatial and temporal control over neural activity in neuronal subsets would greatly enhance our capability to precisely map circuitry function and dysfunction in the brain. Manipulating activity n this way requires a tool that can be genetically targeted to specific populations of neurons and that allows simple and rapid control of neuronal firing. This has been made possible by the recent development of the genetically encoded light-activated cation channel channelrhodopsin-2 (ChR2) for photoactivation and the light-driven chloride pump halorhodopsin (NpHR) for photoinhibition. Recent studies from several laboratories have highlighted the tremendous potentials of using ChR2 and NpHR in mapping neuronal connectivity and manipulating circuitry function. The goal of this research proposal is to generate a series of transgenic mice that express improved NpHR selectively in molecularly defined subtypes of neurons in the brain. Together with our recently generated cell type-specific ChR2 transgenic mice, it will provide a set of powerful genetic tools for interrogating brain circuitry function and dysfunction using high speed photostimulation and photoinhibition in brain slices and in vivo.